1===============================================================================
2WHAT IS EXOFS?
3===============================================================================
4 5exofs is a file system that uses an OSD and exports the API of a normal Linux
6file system. Users access exofs like any other local file system, and exofs
7will in turn issue commands to the local OSD initiator.
8 9OSD is a new T10 command set that views storage devices not as a large/flat
10array of sectors but as a container of objects, each having a length, quota,
11time attributes and more. Each object is addressed by a 64bit ID, and is
12contained in a 64bit ID partition. Each object has associated attributes
13attached to it, which are integral part of the object and provide metadata about
14the object. The standard defines some common obligatory attributes, but user
15attributes can be added as needed.
16 17===============================================================================
18ENVIRONMENT
19===============================================================================
20 21To use this file system, you need to have an object store to run it on. You
22may download a target from:
23http://open-osd.org 24 25See Documentation/scsi/osd.txt for how to setup a working osd environment.
26 27===============================================================================
28USAGE
29===============================================================================
30 311. Download and compile exofs and open-osd initiator:
32 You need an external Kernel source tree or kernel headers from your
33 distribution. (anything based on 2.6.26 or later).
34 35 a. download open-osd including exofs source using:
36 [parent-directory]$ git clone git://git.open-osd.org/open-osd.git
37 38 b. Build the library module like this:
39 [parent-directory]$ make -C KSRC=$(KER_DIR) open-osd
40 41 This will build both the open-osd initiator as well as the exofs kernel
42 module. Use whatever parameters you compiled your Kernel with and
43 $(KER_DIR) above pointing to the Kernel you compile against. See the file
44 open-osd/top-level-Makefile for an example.
45 462. Get the OSD initiator and target set up properly, and login to the target.
47 See Documentation/scsi/osd.txt for farther instructions. Also see ./do-osd
48 for example script that does all these steps.
49 503. Insmod the exofs.ko module:
51 [exofs]$ insmod exofs.ko
52 534. Make sure the directory where you want to mount exists. If not, create it.
54 (For example, mkdir /mnt/exofs)
55 565. At first run you will need to invoke the mkfs.exofs application
57 58 As an example, this will create the file system on:
59 /dev/osd0 partition ID 65536
60 61 mkfs.exofs --pid=65536 --format /dev/osd0
62 63 The --format is optional. If not specified, no OSD_FORMAT will be
64 performed and a clean file system will be created in the specified pid,
65 in the available space of the target. (Use --format=size_in_meg to limit
66 the total LUN space available)
67 68 If pid already exists, it will be deleted and a new one will be created in
69 its place. Be careful.
70 71 An exofs lives inside a single OSD partition. You can create multiple exofs
72 filesystems on the same device using multiple pids.
73 74 (run mkfs.exofs without any parameters for usage help message)
75 766. Mount the file system.
77 78 For example, to mount /dev/osd0, partition ID 0x10000 on /mnt/exofs:
79 80 mount -t exofs -o pid=65536 /dev/osd0 /mnt/exofs/
81 827. For reference (See do-exofs example script):
83 do-exofs start - an example of how to perform the above steps.
84 do-exofs stop - an example of how to unmount the file system.
85 do-exofs format - an example of how to format and mkfs a new exofs.
86 878. Extra compilation flags (uncomment in fs/exofs/Kbuild):
88 CONFIG_EXOFS_DEBUG - for debug messages and extra checks.
89 90===============================================================================
91exofs mount options
92===============================================================================
93Similar to any mount command:
94 mount -t exofs -o exofs_options /dev/osdX mount_exofs_directory
95 96Where:
97 -t exofs: specifies the exofs file system
98 99 /dev/osdX: X is a decimal number. /dev/osdX was created after a successful
100 login into an OSD target.
101 102 mount_exofs_directory: The directory to mount the file system on
103 104 exofs specific options: Options are separated by commas (,)
105 pid=<integer> - The partition number to mount/create as
106 container of the filesystem.
107 This option is mandatory. integer can be
108 Hex by pre-pending an 0x to the number.
109 osdname=<id> - Mount by a device's osdname.
110 osdname is usually a 36 character uuid of the
111 form "d2683732-c906-4ee1-9dbd-c10c27bb40df".
112 It is one of the device's uuid specified in the
113 mkfs.exofs format command.
114 If this option is specified then the /dev/osdX
115 above can be empty and is ignored.
116 to=<integer> - Timeout in ticks for a single command.
117 default is (60 * HZ) [for debugging only]
118 119===============================================================================
120DESIGN
121===============================================================================
122 123* The file system control block (AKA on-disk superblock) resides in an object
124 with a special ID (defined in common.h).
125 Information included in the file system control block is used to fill the
126 in-memory superblock structure at mount time. This object is created before
127 the file system is used by mkexofs.c. It contains information such as:
128 - The file system's magic number
129 - The next inode number to be allocated
130 131* Each file resides in its own object and contains the data (and it will be
132 possible to extend the file over multiple objects, though this has not been
133 implemented yet).
134 135* A directory is treated as a file, and essentially contains a list of <file
136 name, inode #> pairs for files that are found in that directory. The object
137 IDs correspond to the files' inode numbers and will be allocated according to
138 a bitmap (stored in a separate object). Now they are allocated using a
139 counter.
140 141* Each file's control block (AKA on-disk inode) is stored in its object's
142 attributes. This applies to both regular files and other types (directories,
143 device files, symlinks, etc.).
144 145* Credentials are generated per object (inode and superblock) when they are
146 created in memory (read from disk or created). The credential works for all
147 operations and is used as long as the object remains in memory.
148 149* Async OSD operations are used whenever possible, but the target may execute
150 them out of order. The operations that concern us are create, delete,
151 readpage, writepage, update_inode, and truncate. The following pairs of
152 operations should execute in the order written, and we need to prevent them
153 from executing in reverse order:
154 - The following are handled with the OBJ_CREATED and OBJ_2BCREATED
155 flags. OBJ_CREATED is set when we know the object exists on the OSD -
156 in create's callback function, and when we successfully do a
157 read_inode.
158 OBJ_2BCREATED is set in the beginning of the create function, so we
159 know that we should wait.
160 - create/delete: delete should wait until the object is created
161 on the OSD.
162 - create/readpage: readpage should be able to return a page
163 full of zeroes in this case. If there was a write already
164 en-route (i.e. create, writepage, readpage) then the page
165 would be locked, and so it would really be the same as
166 create/writepage.
167 - create/writepage: if writepage is called for a sync write, it
168 should wait until the object is created on the OSD.
169 Otherwise, it should just return.
170 - create/truncate: truncate should wait until the object is
171 created on the OSD.
172 - create/update_inode: update_inode should wait until the
173 object is created on the OSD.
174 - Handled by VFS locks:
175 - readpage/delete: shouldn't happen because of page lock.
176 - writepage/delete: shouldn't happen because of page lock.
177 - readpage/writepage: shouldn't happen because of page lock.
178 179===============================================================================
180LICENSE/COPYRIGHT
181===============================================================================
182The exofs file system is based on ext2 v0.5b (distributed with the Linux kernel
183version 2.6.10). All files include the original copyrights, and the license
184is GPL version 2 (only version 2, as is true for the Linux kernel). The
185Linux kernel can be downloaded from www.kernel.org.
186